Method and apparatus for selectively applying interference cancellation in spread spectrum systems

ABSTRACT

The present invention is directed to the selective provision of interference canceled signal streams to demodulating fingers in a communication receiver. According to the present invention, potential interferer signal paths are identified. Signal streams having one or more potential interferer signals removed or canceled are created, and a correlation is performed to determine whether the strength of a desired signal path increased as a result. If the correlation indicates that the strength of a desired signal path was increased by the signal cancellation, the interference canceled signal stream is provided to the demodulation finger assigned to track the desired signal path. If the correlation determines that the strength of the desired signal path did not increase as a result of performing interference cancellation, the raw or a different interference canceled signal stream is provided to the demodulation finger.

CROSS-REFERENCED TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/502,479, entitled “Method and Apparatus for Selectively ApplyingInterference Cancellation in Spread Spectrum Systems,” and filed Sep.30, 2014, which is a continuation of U.S. patent application Ser. No.13/908,286, entitled “Methods for Estimation and InterferenceSuppression for Signal Processing,” and filed Jun. 3, 2013, now U.S.Pat. No. 8,848,765; which is a continuation of U.S. patent applicationSer. No. 13/205,320, entitled “Methods for Estimation and InterferenceSuppression for Signal Processing,” and filed Aug. 8, 2011, now U.S.Pat. No. 8,457,263; which is a continuation of U.S. patent applicationSer. No. 11/893,707, entitled “Methods for Estimation and InterferenceCancellation for Signal Processing,” and filed Aug. 17, 2007, now U.S.Pat. No. 8,005,128; which (1) claims priority to U.S. Patent ApplicationNo. 60/838,262, entitled “Technique for estimating user and backgroundnoise powers in a code division multiple access system without signalingassistance and application of such to channel quality measurement with alinear receiver,” and filed on Aug. 17, 2006; (2) is acontinuation-in-part of U.S. patent application Ser. No. 11/452,027,entitled “Iterative Interference Cancellation Using Mixed FeedbackWeights and Stabilizing Step Sizes,” and filed Jun. 13, 2006, now U.S.Pat. No. 7,715,508; (3) is a continuation-in-part of U.S. patentapplication Ser. No. 11/432,580, entitled “Interference Cancellation inVariable Codelength Systems for Multi-Access Communication,” and filedMay 11, 2006, now U.S. Pat. No. 7,697,595; (4) is a continuation-in-partof U.S. patent application Ser. No. 11/003,881, entitled “Systems andmethods for serial cancellation,” and filed on Dec. 3, 2004, andpublished as U.S. Patent Application Publication Number 2005-0123080 A1;(5) is a continuation-in-part of U.S. patent application Ser. No.10/686,829, entitled “Method and Apparatus for Channel AmplitudeEstimation and Interference Vector Construction,” and filed on Oct. 15,2003, now U.S. Pat. No. 7,580,448, which claims priority to U.S. PatentApplication No. 60/418,187, entitled “Method for channel amplitudeestimation and interference vector construction,” and filed Oct. 15,2002; and (6) is a continuation-in-part of U.S. patent application Ser.No. 10/669,954, entitled “Method and Apparatus for Selectively ApplyingInterference Cancellation in Spread Spectrum Systems,” and filed on Sep.23, 2003, now U.S. Pat. No. 7,787,518, which claims priority to U.S.Patent Application No. 60/412,550, entitled “Controller for interferencecancellation in spread spectrum systems,” and filed Sep. 23, 2002. Theentirety of each of the foregoing patents, patent applications, andpatent application publications is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention is directed to the application of interferencecancellation in spread spectrum systems. In particular, the presentinvention is directed to selectively applying interference cancellationsuch that if interference cancellation has resulted in an improvedsignal, interference cancellation can be used or continued.

BACKGROUND OF THE INVENTION

Wireless communication systems should provide for a large number ofsecure (or private) communication channels within their allottedfrequency space. In order to achieve these goals, spread spectrumsystems have been developed. In a spread spectrum type system, spreadingcodes are used that allow multiple channels to occupy the same frequencyrange. In order to successfully demodulate a channel, the spreading codeused in connection with the channel must be known. When a demodulationprocessor is tracking a particular signal path, signal paths associatedwith other transmitters appear to that processor as noise.

In order to provide for reliable communications, spread spectrum systemstypically track multiple signal paths in connection with establishingand maintaining a communication channel between a pair of end points.The different signal paths may result from redundant signals that areprovided by additional base stations and base station sectors, or fromreflected or multi-path versions of signals. In a typical receiver, anumber (e.g. 4 to 6) demodulation processors or fingers are provided,and each of these fingers is assigned to track a different signal path.In order to obtain information regarding the different signal paths thatare available to a receiver, a searcher demodulation processor or fingeris provided. In a typical receiver, the searcher finger detects andidentifies signals by pseudorandom number (PN) code offsets and signalstrength. Because signal paths other than the signal path being trackedappear as noise to a demodulation processor, the signal to noise ratiowith respect to a tracked or desired signal path can be low, which canresult in a communication channel with poor quality and reliability. Inparticular, signals from sources that are in close proximity to thereceiver can drown out signals from sources that are farther away fromthe receiver. Accordingly, because of this “near-far” problem, signaldiversity is limited. In addition to leaving communication channels morevulnerable to interruption, relatively weak signals that might otherwisebe available to a receiver lie beneath the noise floor created in theenvironment by other relatively strong signals. This limitation inacquiring and tracking signals from distant sources caused by thenear-far problem also limits the effectiveness of location schemes thatrely on triangulation techniques.

In order to address the near-far problem, schemes have been developedfor controlling the power of signals produced by sources, e.g. beamsteering and smart antenna application. However, such schemes may becomplex and difficult to implement. In addition, where sources such asbase stations are in communication with a large number of receivers,some of which are close to the source and others of which are far fromthe source, the limitation of signal power may not be feasible.

Another approach to allowing receivers to effectively track signalssubject to near-far interference has been to apply interferencecancellation. Such systems remove signal paths that are extraneous fromthe signal path being tracked in a demodulation finger. However, suchsystems have not provided for the flexible application of suchcancellation. As a result, the use of conventional interferencecancellation schemes, as they have heretofore been applied, can actuallyresult in poorer signal to noise ratios with respect to desired signalpaths than if no interference cancellation had been applied.

SUMMARY OF THE INVENTION

The present invention is directed to solving these and other problemsand disadvantages of the prior art. According to the present invention,a method and apparatus for selectively applying interferencecancellation to signals is provided. For example, the present inventionmay apply interference cancellation only if such cancellation results inan improvement in the strength of desired signal paths. Embodiments ofthe present invention also allow for the selection of an interferencecancellation scheme that is determined to be preferred over otherinterference cancellation schemes or over an arrangement in which onlynon-interference cancelled signals are provided to demodulationprocessors or fingers.

According to embodiments of the present invention, the strength of eachof a number of signal paths or identified signals at a receiver isdetermined, and one or more signal paths that are stronger than othersignal paths are identified or determined to contribute a greater amountof interference. According to further embodiments of the presentinvention, signal paths that are not necessarily the strongest but thatnegatively affect another signal path are identified. Cancellation ofsignal paths identified as having a high strength or that negativelyaffect another signal path or paths from signal streams (i.e. raw orinterference cancelled received streams) within a receiver may beinitiated by providing an estimate of a signal path being considered forcancellation to a signal cancellation module. An estimate of the signalpath may be prepared by a channel determination module for the signalcancellation module. The signal cancellation module removes the estimateof the strong potentially interfering signal path from another signalstream or signal streams (i.e., from one or more signal streams that maybe provided to demodulation fingers assigned to track signal paths otherthan the signal path being cancelled). The cancellation controller thendetermines whether the signal to noise ratio of the signal path or pathsderived from the signal stream or streams from which the potentialinterferer have been removed have increased. If an increase in thesignal to noise ratio of a desired signal path is detected, thepotential interferer is identified as an actual interferer, and theinterference canceled version of the desired signal stream may beprovided to the demodulation finger assigned to that desired signalpath.

If an increase in the signal to noise ratio of a desired signal path isnot detected, the interference canceled signal stream will not beprovided to a demodulation finger assigned to that desired signal path.In accordance with another embodiment of the present invention, aninterference canceled signal stream is not provided to a demodulationfinger assigned to a desired signal path unless it has been determinedthat the interference canceled signal stream will likely result in anincrease in the strength of the desired signal by at least a thresholdamount. In accordance with embodiments of the present invention, theanalysis of the effect of providing different interference canceledsignals to demodulation fingers can proceed such that either the rawsignal stream or an interference canceled signal stream is identified asproviding the greatest signal strength with respect to a desired signalpath. This can be done for each signal path assigned to a demodulatingfinger provided by a receiver. In accordance with another embodiment ofthe present invention, signal cancellation may be achieved using variousmethods. For example, a replica of the potential interferer oridentified interferer may be subtracted from the raw signal stream. Inaccordance with another embodiment of the present invention, serialcancellation techniques using projection-based methods of removingpotential interferers or identified interferers may be used. Inaccordance with still other embodiments of the present invention,parallel cancellation of potential interferers and identifiedinterferers may be applied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting components of a spread spectrumreceiver in accordance with the prior art;

FIG. 2 is a block diagram of components of a spread spectrum receiver inaccordance with an embodiment of the present invention;

FIG. 3 is a diagram depicting the flow of signals through a spreadspectrum receiver in accordance with an embodiment of the presentinvention;

FIG. 4 is a schematic depiction of signal and information flows througha spread spectrum receiver in accordance with an embodiment of thepresent invention;

FIG. 5 is a flowchart illustrating aspects of a controller operatingcycle in accordance with an embodiment of the present invention;

FIG. 6 is a flowchart illustrating aspects of the operation of a spreadspectrum receiver in accordance with an embodiment of the presentinvention;

FIG. 7 is a flowchart illustrating a process for updating a to cancellist in accordance with an embodiment of the present invention;

FIG. 8 is a flowchart illustrating a process for adding signal paths toa to cancel list in accordance with an embodiment of the presentinvention;

FIG. 9 is a flowchart illustrating a process for updating a survey pathlist in accordance with an embodiment of the present invention;

FIG. 10 is a flowchart illustrating a process for removing a path from ato cancel list in accordance with an embodiment of the presentinvention;

FIG. 11 is a flowchart illustrating a process for controlling the signalflow to a demodulation finger in accordance with an embodiment of thepresent invention;

FIG. 12 illustrates the contents of a candidate to cancel list inaccordance with an embodiment of the present invention;

FIG. 13 illustrates the contents of a to cancel list in accordance withan embodiment of the present invention;

FIG. 14 illustrates the contents of a canceled paths list in accordancewith an embodiment of the present invention; and

FIG. 15 illustrates the contents of a canceled signal feed list inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

With reference now to FIG. 1, components of a baseline or prior artspread spectrum communication receiver 100 are illustrated. As depictedin FIG. 1, signals are provided to a radio frequency front end 104 by anantenna 108. In a typical environment, a number of different signals,for example, signals produced by different base stations, differentsectors of a base station, or multi-path or reflected versions of thesignals can be received at the radio frequency front end 104. As can beappreciated by one of skill in the art, signals from different basestations or different sectors of a base station are typically identifiedby an associated path number, which identifies the base station or basestation and sector according to the time offset of the signal path.Multi-path versions of signals are identified by the path number of theline of sight version of the signal, plus an additional time offset toaccount for the longer path followed by the reflected signal. As canfurther be appreciated by one of skill in the art, signal paths fromdifferent sources are typically separated by a distance (e.g., 64 chips)sufficient to allow the source of multi-path versions of signal paths tobe correctly identified with the source of such signal paths.

The raw signal stream 112 collected by the receiver 100 anddown-converted by the RF front end 104 is provided to a searcher finger116. The searcher finger functions to scan the signal stream 112 forindividually identifiable signal paths and/or multi-paths. Inparticular, the searcher finger 116 operates to determine the pathnumber or pseudorandom number (PN) code offset associated with eachidentifiable signal path. As noted above, the PN code identifies thissignal path as being associated with a particular base station or basestation sector. In code division multiple access (CDMA) systems, the PNcode sequence is referred to as the short code.

The searcher finger 116 reports the signal paths that have beenidentified to the controller 120. Information provided to the controllermay be placed in a survey path list. In general, the survey path listidentifies by PN offsets those signal paths that are visible to thesearcher finger 116. Alternatively, the survey path list may contain thePN offsets for those signal paths that have at least a threshold signalto noise ratio or strength.

The controller 120 reports the identities of the signal paths to thesurvey path list. From the survey path list, the controller 120 maydecide to acquire and track one or more of the signal paths on thesurvey path list. In general, the number of signal paths that a receiver100 can be directed to track is limited by the number of demodulationfingers 124 provided as part of the receiver 100. The signal pathsassigned to the receiver 100 for demodulation and tracking may beprovided as a demodulation path list. In a typical communication system,the demodulation path list comprises an identifier for each demodulationfinger 124, an identifier of the signal path assigned to eachdemodulation finger 124, any additional time offset, the observedstrength of the signal and the sector of the signal path.

The demodulation fingers 124 receive as a feed signal the raw signalstream 112 from the radio frequency front end 104, and each acquires thesignal path assigned to that finger 124, as set forth in thedemodulation path list. The demodulated signal stream is then providedto a symbol combiner 132, which combines the demodulated signal streams128 provided by the demodulation fingers 124. For example, the signalcombiner 132 and the demodulation fingers 124 collectively comprise arake receiver. Although the receiver 100 depicted in FIG. 1 illustratesfirst 124 a and second 124 b demodulation fingers, and associated first128 a and second 128 b demodulated signals, receivers 100 having varyingnumbers of demodulation fingers 124 have been developed. For example,commercially available CDMA telephones commonly have from 4 to 6demodulation fingers 124.

The prior art receiver 100 illustrated in FIG. 1 does not include aninterference canceller for providing an interference canceled signalstream to the demodulation fingers 124. Receivers 100 that providesignal cancellation have been developed. However, such systems have notprovided for selective application of interference cancellation.Accordingly, such systems have applied signal cancellation withoutregard to whether such cancellation actually results in improvements tothe signal strengths of desired signal paths. Accordingly, such systemscan actually reduce the observed signal strength of desired signalpaths.

With reference now to FIG. 2, a cancellation controlled receiver 200 inaccordance with an embodiment of the present invention is illustrated.In general, the receiver 200 provides for the selective application ofinterference cancellation. For example, a receiver 200 in accordancewith the present invention is capable of determining whether aninterference canceled signal stream will improve the observed strengthof a desired signal path in the receiver 200. Furthermore, such adetermination can be made before an interference canceled signal streamis provided to a demodulation finger. In addition, the receiver 200 canprovide a different interference canceled signal stream, or a signalstream that has not undergone interference cancellation, if it isdetermined that a previously selected interference canceled signalstream is no longer resulting in an improved signal to noise ratio withrespect to a desired signal path.

In general, the receiver 200 includes a radio frequency (RF) front end204 and associated antenna 208. The raw signal stream 212 collected bythe RF front end 204 and antenna 208 is provided to a searcher finger216. The searcher finger 216 may operate to locate signals within theraw signal stream 212. The identity of the signal paths located by thesearcher finger may then be reported to a baseline controller 220. Inparticular, information regarding signal paths located by the searcherfinger 216 may be used to construct a survey path list. The baselinecontroller 220 assigns the receiver 200 to track all or a selected setof the signal paths identified in the survey path list. The assignmentof signal paths that are to be acquired and tracked by demodulationfingers 224 in the receiver 200 may be performed in association with ademodulation path list. In FIG. 2, only two demodulation fingers 224 aand 224 b are shown. However, it should be appreciated that any numberof additional demodulation fingers 224 may be provided.

As shown in FIG. 2, the baseline controller 220 may be in communicationwith a cancellation controller 228. The cancellation controller 228 isunique to the present invention. In general, and as will be described ingreater detail elsewhere herein, the cancellation controller 228controls the production and selection of interference canceled signals.As part of the selection process, the cancellation controller 228 iscapable of determining that interference canceled signal streamsavailable to the receiver 200 do not result in improving the signal tonoise ratio of a desired signal path, and may therefore direct theprovision of the raw signal stream 212 as a feed signal stream to all orsome of the demodulation fingers 224.

As shown in FIG. 2, the cancellation controller 228 may include one ormore channel determination modules 232 and one or more signalcancellation modules 236. In general, the channel determination modules232 operate to produce replicas of signal paths that are identified bythe cancellation controller 228 as being potential or actual interferersignal paths. The signal cancellation modules 236 receive the replicasignal paths, and perform cancellation to remove such signals from theraw signal stream 212. The cancellation controller may also include oneor more correlators 238 for determining whether interference canceledsignal streams produced in the signal cancellation modules 236 provide adesired signal path having an increased strength.

In order to provide either the raw (or baseline) signal stream or aninterference canceled signal stream to a demodulation finger 224, asignal line 244 a and 244 b for carrying the signal stream is providedbetween the cancellation controller 228 and the correspondingdemodulation fingers 224 a and 224 b. In addition, control signal paths240 are provided between the cancellation controller and thedemodulation fingers 224 to control the delay or advance of the PN codesby the PN generator 246 associated with each demodulation finger 224.The ability to delay or advance the PN generators 246 associated withthe demodulation fingers 224 is advantageous because it allows thedemodulation fingers 224 to each track a provided feed signal stream244, even if that feed signal stream 244 has undergone delays, forexample in processing in the cancellation controller 228. Thecancellation controller 228 may also provide a demodulated signal delaycontrol signal 248 to a delay buffer 252 to control an amount of delayintroduced by each demodulation finger 224 before a symbol obtained fromthe provided signal stream 244 is made available to a symbol combiner260. By so controlling the delay within the demodulation fingers 224,demodulated signal streams 256 can be synchronized by the cancellationcontroller 228. Accordingly, a conventional symbol combiner 260 may beused. Alternatively, a symbol combiner 260 that is capable ofsynchronizing symbols obtained from the processing of signal streams 244by the demodulation fingers 224 may be used, in which case the delaybuffers 252 and associated signal lines could be omitted. As yet anotheralternative, the cancellation controller 228 may provide feed signalstreams to demodulating fingers 224 after a fixed delay with respect tothe raw signal stream 212 as it is received in the RF front end 204 so aconventional combiner 260 may be used.

In accordance with an embodiment of the present invention, the receiver200 may also provide an interference canceled signal connection 264capable of delivering an interference canceled signal stream from thecancellation controller 228 to the searcher finger 216. Such anembodiment allows the searcher finger 216 to scan interference canceledversions of the raw signal stream 212 for available signal paths.Accordingly, signal paths that may have been buried beneath the noisefloor in a raw signal stream 212 may become visible to the searcherfinger 216 in an interference canceled signal stream. Accordingly, agreater number of signal paths can be made available to the receiver 200for acquisition and tracking by the provided demodulation fingers 224,which can increase the reliability and quality of a communicationchannel. In addition, by potentially making signal paths originating atadditional base stations visible to the receiver 200, locationtechnologies that utilize triangulation techniques between differentsignal sources and the receiver 200 can provide a more accurate locationdetermination.

In connection with an embodiment in which the searcher finger 216 may bedirected to scan interference canceled signal streams, the cancellationcontroller 228 may operate to provide PN code delay information to thesearcher finger 216. Such information allows the searcher finger 216 toaccurately identify the PN of signal paths, even though the feed signalstreams will have been delayed by the process of creating theinterference canceled signal stream.

With reference now to FIG. 3, signal flows within a receiver 200 inaccordance with an embodiment of the present invention are illustrated.As shown in FIG. 3, the flow of signal streams through the receiver 200begins with the receipt of the raw data signal stream 212. The rawsignal stream is provided to the searcher finger 216, and also to thedemodulation fingers 224. The demodulation fingers 224 each produce ademodulated signal stream 304. In FIG. 3, three demodulated signalstreams 304 a, 304 b and 304 n are depicted. Accordingly, FIG. 3 wouldcorrespond to a receiver 200 having at least three demodulation fingers224. As can be appreciated by one of skill in the art, the depiction ofthree demodulated signal streams 304 is representative. In particular, areceiver 200 may produce a greater or lesser number of demodulatedsignal streams, provided that an appropriate number of demodulationfingers 224 are available.

The demodulated signal streams 304 are provided to the channeldetermination modules 232 of the cancellation controller 228. As will bedescribed in greater detail elsewhere herein, the channel determinationmodules 232 produce estimates of potential interfering signals 308. InFIG. 3, estimates 308 a, 308 b and 308 n are shown. Accordingly, anembodiment producing the data flow illustrated in FIG. 3 might havethree channel determination modules 232. However, it should beappreciated that the number of channel determination modules 232 is notrequired to match the number of demodulation fingers 224 provided by areceiver 200.

The estimates of potential interfering signal paths 308 are provided tothe signal cancellation modules 236 of the cancellation controller 228.The signal cancellation modules remove the interfering signal paths fromone or more of the feed signal streams provided to the demodulationfingers 224. For example, the estimate 308 a of the first demodulatedsignal path 304 a may be removed from the signal stream that will beprovided to the demodulation finger 224 assigned to track the secondsignal path and/or the demodulation finger 224 assigned to track the nthsignal path. The estimate of the nth signal path may also be removedfrom either or both of the feed signal streams provided to thedemodulation fingers 224 assigned to track the first and second desiredsignal paths. Likewise, the estimate of the second interfering signalpath 308 b may be removed from either or both of the signal streamsprovided to the demodulating fingers 224 assigned to track the first andnth signals path. The cancellation controller 228 then checks thestrength of the interference canceled signal path 312 a-n for theassigned signal paths. If it is determined that the strength, forexample as represented by an observed signal to noise ratio, hasincreased for an interference canceled signal path, that interferencecanceled signal stream may be provided to the assigned demodulationfinger. If it is determined that the strength of a desired signal pathhas not been increased through the use of an interference canceled feedsignal stream, the interference canceled signal stream 312 underconsideration is not sent to the demodulating finger 224 assigned totrack the signal path under consideration. Instead, another signalstream, such as the raw signal stream 212, or a previous version of aninterference canceled signal stream 312 having a different signal or setof signals cancelled therefrom, may be provided as the feed signalstream to the demodulating finger 224.

With reference now to FIG. 4, the flow of signal streams through areceiver 200 in accordance with an embodiment of the present invention,showing the use of various lists or tables and their relationship tofunctional elements, is illustrated. Initially, a raw signal stream 212is provided as a feed signal stream to the searcher finger 216. Thesearcher finger scans the raw data signal stream 212 to locate andidentify signal paths available to the receiver 200. As shown in FIG. 4,an interference canceled signal stream 264 may, in accordance with someembodiments of the present invention, be provided in addition or as analternative to the raw data signal stream 212. As noted above, thesearcher finger 216 provides the information identifying the availablesignal paths to the baseline controller 220. The baseline controller 220uses the information provided by the searcher finger 216 to prepare asurvey path list 404. The survey path list generally includes the PNoffsets of signals located by the searcher finger 216. The baselinecontroller 220 uses the information from the survey path list to assignsignal paths to be tracked by the receiver 200 in available fingers.These assignments are recorded in the demodulation path list 412.Accordingly, the demodulation path list 412 may identify the fingersavailable in the receiver 200, the signal path assigned to each finger,any time offset, the signal strength of each signal path, and the sectorof the signal path. The demodulation path list 412 is then madeavailable to the cancellation controller 228.

The cancellation controller 228 checks for the presence of interferingsignal paths in the demodulation path list 412. For example, inaccordance with an embodiment of the present invention, the cancellationcontroller 228 may check for signal paths being tracked by ademodulation finger 224, and that therefore are listed in thedemodulation path list 412, that have a signal strength that is greaterthan a predetermined threshold. The strength of the signal paths may bedetermined by measuring the signal to noise ratio of the signal path, ora selected component or channel of the signal path. For example, inaccordance with an embodiment of the present invention, the strength ofthe pilot channel signal transmitted by a base station may be measuredto determine the strength of the signal paths. In accordance withanother embodiment of the present invention, some or all of the trafficchannels, the pilot channel, the paging channel, and/or thesynchronization channel may be used to determine the strength of asignal path. As can be appreciated by one of skill in the art, greateraccuracy in measuring the strength of a signal path can be realized ifall or a significant number of the channels or signals within a signalpath are measured. However, monitoring a large number of the channelsand/or signals within a signal path is computationally expensive.Accordingly, embodiments of the present invention can monitor arelatively small number of the channels or signals within a signal path.For instance, the strength of the pilot signal alone may be measured todetermine the strength of the associated signal path. For example,estimates of signal strengths can be determined from the E_(e)/I_(o),where E_(e) is energy per chip and I_(o) is the total power orinterference in the system, can be used to provide a signal to noiseratio value for estimating the strength of the signal path.

Signal paths meeting the criteria for a potential interfering signalpath are identified in a candidate to cancel list or table 416. Withreference now to FIG. 12, the elements of an exemplary candidate tocancel list 416 are illustrated. As shown in FIG. 12, the candidate tocancel list 416 may include entries identifying demodulating fingers224, the signal paths 1204 assigned to each demodulating finger 224, thesignal strength 1208 of the signal path, and the sector 1212 of thesignal path. As can be appreciated by one of skill in the art, eachsignal path may comprise or reference a different PN code offset ormultipath version of a PN code offset.

The strongest signal paths included in the candidate to cancel list 416are assigned to a to cancel list or table 420. As illustrated in FIG.13, in accordance with an embodiment of the present invention, the tocancel list 420 contains a maximum of n signal paths, where ncorresponds to the number of channel determination modules 232 providedby the cancellation controller 228. Accordingly, where the number ofpotential interfering signal paths exceeds the number of channeldetermination modules 232, the to cancel list 416 may include the nstrongest potential interfering signal paths. With reference now to FIG.13, the contents of a to cancel list in accordance with an embodiment ofthe present invention is illustrated. In general, the to cancel list 420includes an entry for a demodulating finger 224 and a correspondingsignal path 1304. In particular, the to cancel list 420 identifiessignal paths, that are to be canceled.

With continued reference to FIG. 4, using the information included inthe to cancel list 420, the cancellation controller 228 operates thechannel determination modules 232 to provide outputs from one or more ofthe demodulation fingers 224 to the appropriate channel determinationmodule or modules 232. Optionally, the information regarding thepairings of the output from demodulation fingers 224 to channeldetermination modules 232 may be maintained in a channel determinationlist or table 424, which generally contains the same information as theto cancel list 420. By providing the appropriate signal streams to thechannel determination modules 232, interference canceled signal streamsare available at the output of the signal cancellation modules 236. Thecancellation controller 228 may further implement or controlmultiplexers 428 for selecting an output available from a signalcancellation module 236 or a raw signal stream 212 to be provided in thereceiver 200 as a feed signal stream to a demodulation finger 224 or toa correlator 238 provided as part of the cancellation controller 228.The cancellation controller 228 then determines whether the interferencecanceled signal streams provided to some or all of the demodulationfingers 224 or correlators 238 has resulted in an improved desiredsignal strength. In accordance with an embodiment of the presentinvention, this determination is made by correlating desired signalpaths with the raw signal stream 212 and the interference canceledsignal stream provided to the corresponding demodulation finger 224. Thecorrelator implemented by the cancellation controller 228 may furthercomprise a bank of correlators 238.

In accordance with an embodiment of the present invention, a correlator238 operates by performing a vector inner product or correlationoperation: x^(T)y, where x is a reference signal, such as a pilotsignal, ^(T) is the transpose operation, and y is a feed signal stream.Therefore, whether the use of an interference cancelled signal y₁results in an improved signal strength can be determined comparing theresult of x^(T)y₁ to the result of x^(T)y_(raw), where y_(raw) is anon-interference cancelled signal stream. The reference signal x, mayconsists of nothing but a series of 1's and −1's, e.g. the short code orPN sequence. This reference signal may consist of a replica of the pilotchannel, which is a non-information bearing channel. The result of thevector inner product can then be used to determine the strength of thecorrelation between the received signal stream and the PN sequence,because the PN sequence is known. As still another example, signalcancellation could be performed with respect to an interferencecancelled version of x and y_(raw). In accordance with still anotherembodiment of the present invention, a look-up table can be used inplace of a correlation operation. In particular, using informationregarding the relative signal to noise ratios of a raw signal path andan interference cancelled signal path, previously calculated valuesstored in a look-up table can be referenced in order to estimate whetherit would be preferable to use an interference cancelled signal stream ora non-interference cancelled signal stream.

Those signal paths, the cancellation of which resulted in improvedsignal strengths for a desired signal path, are listed in a canceledpaths list or table 432 (FIG. 4). FIG. 14 illustrates the contents of acanceled paths list 432 in accordance with an embodiment of the presentinvention. As shown in FIG. 14, the canceled paths list 432 may indicatethose signal paths that have been canceled from a signal stream providedto a demodulation finger 224. The canceled paths list 432 is thencompared to the demodulation path list 412.

If a signal path is present on both the demodulation path list 412 andthe canceled paths list 432, a canceled signal feed list or table 436 isupdated to indicate that the feed to one or more demodulation fingers224 comprises an interference canceled signal stream, rather than a rawsignal stream. With reference now to FIG. 15, a canceled signal feedlist 436 in accordance with an embodiment of the present invention isillustrated. As shown in FIG. 15, the canceled signal feed list maycontain a list of demodulation fingers 224. For each demodulation finger224 in the canceled signal feed list 436, the identity of a signalcancellation module 236 and canceled signal path 1504 may be indicated.If a demodulation finger 224 is not listed in the canceled signal feedlist 436 as receiving an interference canceled signal stream, it isprovided with the raw signal stream 212.

With reference now to FIG. 5, a channel determination cycle inaccordance with an embodiment of the present invention is illustrated.Initially, at step 500, a demodulation path list 412 is created ormodified. At step 504, the demodulation path list is read, and at step508 the strong potential interferers are identified. Next, at step 512,interference canceled signal streams are created. In particular, signalstreams from which one or more strong potential interfering signal pathshave been removed are created.

At step 516, the interference canceled signal streams are correlatedwith the reference signal and the raw data signal 212 correlated withthe reference signal is computed. A determination is then made as towhether the signal to noise ratio (i.e., the strength) of a desiredsignal at the output of a corresponding correlator 238 has improved(step 520). If the strength of the desired signal path has improved, theinterference canceled version of the input signal stream is provided tothe demodulation finger 224 for use in connection with communicationsinvolving the receiver 200 (step 524). If the strength of the signalpath from the finger 224 has not improved, the interference canceledversion of the signal stream is not provided to the finger 224. Instead,the raw signal stream is provided to the finger 224.

At step 528, a determination is made as to whether there are more signalstreams to be considered. If signal streams remain to be considered, thenext signal is obtained (step 532), and the system returns to step 520.In this way, the effect of providing an interference canceled signalstream to each demodulation finger 224 in a receiver 200 is assessed. Ifno more signal streams remain to be considered the channel determinationcycle ends (step 536). As can be appreciated by one of skill in the art,the channel determination cycle may start again the next time that thesignal paths to be tracked by the demodulating fingers 224 change. Forexample, the channel determination cycle may start again when thedemodulation path list 412 is created or modified.

With reference now to FIG. 6, the operation of a receiver 200 inaccordance with an embodiment of the present invention is illustrated ingreater detail. As shown in FIG. 6, at step 600 the demodulation pathlist, listing demodulating fingers 224 and the signal path assigned toeach finger, is obtained. At step 604, the signal paths that arepotential interferers are identified. For example, in accordance with anembodiment of the present invention, those signals paths having a signalstrength that is greater than a predetermined threshold are identified.The identified potentially interfering signal paths are then stored inthe candidate to cancel list 416 (step 608). From the candidate tocancel list, up to n signal paths are assigned to the to cancel list 420(step 612).

At step 616, the output of each demodulation finger 224 is connected toa corresponding channel determination module 232, and estimates of theinterfering signal paths are produced. In accordance with an embodimentof the present invention, the estimate of the interfering signal pathcomprises a replica of that signal path, where the signal cancellationimplemented by the signal cancellation modules 236 uses subtractivecancellation. In accordance with another embodiment of the presentinvention, the estimate of an interfering signal path is expressed as avector or matrix as described in U.S. patent application Ser. No.10/294,834, filed Nov. 15, 2002, the entire disclosure of which isincorporated herein by reference, for use in a serial cancellationinterference cancellation design, as described in U.S. patentapplication Ser. No. 10/247,836, filed Sep. 20, 2002, the entiredisclosure of which is incorporated herein by reference, or for use in aparallel type signal cancellation arrangement as described in U.S.Patent Application Ser. No. 60/445,243, filed Feb. 6, 2003, the entiredisclosure of which is also incorporated herein by reference, where anon-orthogonal projection of an interfering signal path is made tocancel the interference. In accordance with another embodiment, inaddition or as an alternative to such non-orthogonal projectiontechniques, orthogonal projection techniques may be used. In general,any suitable noise or signal cancellation technique can be used inconnection with the selection process provided in connection withembodiments of the present invention. In accordance with an embodimentof the present invention, the replica of the interfering signal path isproduced by monitoring one or more Walsh code channels present in thesignal path to be canceled. Accordingly, a signal path identified as aninterfering signal path, and therefore a signal path that is to becanceled from other signal streams, must be tracked within at least oneof the demodulation fingers 224 in order to build a replica signal.Moreover, by tracking the signal path to be canceled in one of thefingers, correlation can be performed periodically to determine thepower of that signal and the amount of interference that it contributes.

At step 620, the signal cancellation modules 236 remove the interferingsignal path from a feed signal stream using the estimates from thechannel determination modules 232. The cancellation controller 228 thenchecks the strength of the resulting signal paths, and adds signal pathshaving increased strength to the canceled paths list 432 (see FIG. 4).As can be appreciated by one of skill in the art, more than oneinterference canceled stream may provide a benefit to a signal path, inwhich case a choice must be made between more than one signal streamsthat provide a benefit. Such choice can be made by ranking the estimatedeffects of providing the various signal streams. Then, the survey pathlist 404 may be updated to reflect the new signal strength for themonitored signal paths. Accordingly, a number of different interferencecancelled signal streams and the raw signal stream may be evaluated withrespect to the reception of one or more signal paths.

At step 628, a determination is made as to whether the demodulation pathlist 412 has been updated. If new paths have been assigned, thedemodulation path list 412 is updated (step 632). If new paths have notbeen assigned to the demodulation path list, or after updating thedemodulation path list 412, the canceled path list 432 and demodulationpath list 412 are compared, and the appropriate interference canceledsignal stream is sent to the corresponding finger 424 if that signalpath is present on both lists (step 636). That is, if a signal path ispresent on both lists, it is being tracked by a demodulation finger 224,and thus an estimate of that signal path can be prepared, and it hasbeen identified as an interfering signal path with respect to at leastone other signal path being tracked within the receiver 200. Thecanceled signal feed list 436 (see FIG. 4) is then updated to indicatethe assignment of interference canceled signal streams to demodulationfingers 224 (step 640).

With reference now to FIG. 7, a process for updating a to cancel list420 in accordance with an embodiment of the present invention isillustrated. Initially, at step 700, the cancellation controller 228initiates a check of the to cancel list 420. At step 704, a count valuep is set equal to the first element (corresponding to a desired signalpath) in the to cancel list 420. Next, a determination is made as towhether element p appears in the candidate to cancel list 416 (step708).

If p appears in the candidate to cancel list 416, the signal strengthfor path p in the to cancel list 420 is, updated, and element p isremoved from the candidate to cancel list 416 (step 712). Adetermination is then made as to whether there are more paths in the tocancel list 420 to be considered (step 716). If there are other paths inthe to cancel list 416, p is set equal to the next element in the tocancel list 416 (step 720). If there are no additional paths in the tocancel list 416, the process of updating the cancel list 416 ends (step724).

If at step 708 it is determined that p is not in the candidate to cancellist 416, path p is removed from the to cancel list 420 (step 728).Channel determination is initiated, and cancellation of path p isdisabled (step 732). The process then proceeds to step 716 to determinewhether there are more paths in the to cancel list 420 to consider.

With reference now to FIG. 8, a process for adding signal paths to a tocancel list 420 in accordance with an embodiment of the presentinvention is illustrated. Initially, at step 800, p is set equal to thefirst path in the candidate to cancel list 416. At step 804, path p isadded to the to cancel list 420. An update of channel determination isthen performed, and appropriate connections to channel determinationmodules are enabled (step 808). At step 81 determination is made as towhether there are additional paths in the candidate to cancel list 416.If there are additional paths, p is set equal to the next element in thecandidate to cancel list 416 (step 816) and the process returns to step804. If there are no additional paths in the candidate to cancel list416, the process may proceed to update the survey path list 404 (step820).

With reference now to FIG. 9, a process for updating a survey pathand/or cancel path list in accordance with an embodiment of the presentinvention is illustrated. In general, this process is entered while theto cancel list 420 is not null (i.e., while there are signals listed forcancellation) (step 900). At step 904, s(i) is set equal to the firstelement in the to cancel list 420. The searcher finger 216 is thencommanded to check the interference canceled signal (i) for thestrengths of all PN offsets in the survey paths (step 908).Alternatively, the interference cancelled signal is sent to a bank ofcorrelators 238. The survey path list 404 is then updated for any PNoffsets that show a signal to noise ratio improvement greater than somethreshold amount (step 912). In accordance with an embodiment of thepresent invention, the survey path list 404 maybe updated for any PNoffsets that show any improvement. It should be appreciated thatupdating the survey path is optional. In particular, the survey pathlist may be updated if an interference canceled signal stream is sent tothe searcher or by some other means. However, updating the survey pathlist is not necessary in other circumstances for a cancellationcontroller in accordance with embodiments of the present invention towork.

At step 916, any updated paths are added to the canceled path list 432.Such paths are then removed from any previous canceled path (i−1) list(step 920). A determination is then made as to whether there areadditional elements in the to cancel list 420. If elements remain in theto cancel list 420, s(i) is set equal to the next element in the tocancel list 420, and the process returns to step 908. If there are nomore elements in the to cancel list 420, the process for updating thesurvey path list 404 ends (step 932).

With reference now to FIG. 10, a process for removing a path from a tocancel list 420 in accordance with an embodiment of the presentinvention is illustrated. Initially, at step 1000, the cancellationcontroller 238 generates a signal to de-assign element p from the tocancel list 420. Thus, at step 1004, the corresponding entry in the tocancel list 420 is nulled. The flow of data from the finger 224 trackingthe signal associated with element p to the channel determination module232 is disconnected and the state of the channel determination module232 is reset (step 1008). The corresponding signal cancellation module236 is disabled and is also reset (step 1012). At step 1016, the PNcodes in the demodulation fingers 224 are then advanced or slewedaccordingly in order to synchronize with the baseline or raw signal 212.The process for removing a path from the to cancel list then ends (step1020).

With reference now to FIG. 11, a process for controlling the signal flowto a demodulation finger in accordance with an embodiment of the presentinvention is illustrated. Initially, at step 1104, the cancellationcontroller 228 signals that signal path p is to be added to the canceledsignal feed list (i) 436. At step 1108, the demodulating finger 224 forpath p is determined. At step 1112, a signal stream from which signalpath p has been canceled is provided to the finger assigned to receivethat interference cancelled signal stream (as shown in the canceledsignal feed list 436). The PN generator in the demodulating finger 224associated with the canceled signal feed list 436 is then delayed orslewed to synchronize with the interference canceled signal stream nowbeing provided (step 1116). The process for controlling the signal flowto a demodulation finger then ends (step 1120).

As can be appreciated by one of skill in the art, the present inventionprovides a method and apparatus for selectively applying interferencecancellation. In particular, the present invention allows either theprovision of an interference cancelled signal stream or anon-interference cancelled signal stream to a demodulation finger, inorder to provide the most favorable signal to noise ratio. It shouldfurther be appreciated that the present invention can be used inconnection with any existing or newly developed signal cancellationprocedure or mechanism to selectively apply such signal cancellation. Inparticular, by considering the effect or estimated effect of differentsignal streams on the reception of a desired signal path, the raw orinterference cancelled signal streams providing a more favorablereception of a desired signal path can be selected. In particular, byallowing interference cancelled signal streams to be selectivelyapplied, the present invention can avoid obtaining a degraded signal tonoise ratio for a desired signal path as a result of the blindapplication of an interference cancelled signal stream. Specifically,the present invention provides a method and apparatus by which apreferred feed signal stream can be identified and provided to ademodulating, finger.

Although the description provided herein has at times used examples ofreceivers comprising cellular telephones in spread spectrum systems, itshould be appreciated that the present invention is not so limited. Inparticular, the present invention may be applied to any wirelesscommunication system component implementing a wireless link or channelcapable of using a plurality of channels substantially simultaneously.Accordingly, the present invention may be used in both mobile devices,such as telephones or other communication endpoints, or in wireless basestations or nodes. Furthermore, the present invention is not limited toterrestrial applications. For example, the present invention may be usedin connection with satellite communication systems. In addition, thepresent invention is not limited to voice communication systems. Forexample, embodiments of the present invention may be applied to anymultiple channel system, including radio locating systems, such as theglobal positioning system (GPS), multimedia communications, and datatransmission systems.

The foregoing discussion of the invention has been presented forpurposes of illustration and description. Further, the description isnot intended to limit the invention to the form disclosed herein.Consequently, variations and modifications commensurate with the aboveteachings, within the skill and knowledge of the relevant art, arewithin the scope of the present invention. The embodiments describedhereinabove are further intended to explain the best mode presentlyknown of practicing the invention and to enable others skilled in theart to utilize the invention in such or in other embodiments and withvarious modifications required by their particular application or use ofthe invention. It is intended that the appended claims be construed toinclude the alternative embodiments to the extent permitted by the priorart.

The invention claimed is:
 1. A method comprising: determining a signalstrength for each of a plurality of signal paths; identifying a givenone of the plurality of signal paths as a potential interferer based, atleast in part, on the signal strength of the given one of the pluralityof signal paths; and creating at least one interference-cancelled signalstream by canceling the potential interferer.
 2. The method of claim 1,further comprising: determining whether providing the at least oneinterference-cancelled signal stream to at least one signal processorwill improve a signal-to-noise ratio (SNR) of at least one signal pathassigned to the at least one signal processor; and in response todetermining that the at least one interference-cancelled signal streamwill improve the SNR of the at least one signal path assigned to the atleast one signal processor, providing the at least oneinterference-cancelled signal stream to the at least one signalprocessor.
 3. The method of claim 1, further comprising: determiningwhether providing the at least one interference-cancelled signal streamto at least one signal processor will improve a signal-to-noise ratio(SNR) of at least one signal path assigned to the at least one signalprocessor; and in response to determining that the at least oneinterference-cancelled signal stream will not improve the SNR of the atleast one signal path assigned to the at least one signal processor,discontinuing said creating the at least one interference-cancelledsignal stream.
 4. The method of claim 1, wherein the plurality of signalpaths comprises a plurality of assigned signal paths.
 5. The method ofclaim 4, wherein the plurality of assigned signal paths is obtained froma demodulation path list.
 6. The method of claim 1, further comprising:listing the potential interferer in a candidate-to-cancel list includinga plurality of signal paths; and updating the candidate-to-cancel listby replacing a signal path having a signal strength that is less than asignal strength of the potential interferer with the potentialinterferer.
 7. The method of claim 1, further comprising: storing anidentity of the plurality of signal paths to create a survey path list;providing an interference-canceled signal stream to at least one of asearcher element or a correlator element; and updating the survey pathlist.